The invention relates to a cannula, especially a dialysis cannula, with an obliquely sharpened point.
Cannulas with an obliquely and acutely sharpened point are usually used in medical practice in puncturing body cavities and blood vessels for the purpose of withdrawing or introducing body fluid (e.g. blood) or medicines. The structural design of the cannula, especially of the cannula point, is of particular importance, as is the technique of the manufacturing process in the case of so-called dialysis cannulas. The latter are cannulas which have to be repeatedly introduced into blood vessels of a patient with continual kidney failure for the purpose of connection to the artificial kidney. Each time the patient receives artificial kidney treatment (usually three times a week for life) two cannulas have to be introduced into one of the patient's blood vessels which has been specially surgically prepared before the beginning of the first treatment. The cannulas then remain in the blood vessel for four to eight hours for the duration of a single treatment. The cannulas impede the patient's freedom of movement and involve the danger of a perforating injury to the vessel wall. The latter is particularly important since the patient's blood must be made temporarily incoagulable during extrabodily circulation in order to prevent the formation of clots.
Dialysis cannulas have an essentially larger outer diameter (approximately 1.6 to 2.2 mm) than the usual injection cannulas. Cannulas with ultrathin wall thicknesses (0.05 to 0.1 mm) are also manufactured at present, in order to obtain the smallest possible outer diameter with the same inner diameter.
Cannulas can now be sharpened such that relatively small forces, which act on the cannula point during puncture, are perceptible and contribute to the success of the puncture attempt.
As the treatment procedure with the artificial kidney is, in the long run, the most expensive medical therapy, the manufacturing costs of dialysis cannulas must of course be favorable. A further consideration is that the patient suffering with a chronic kidney disease can only survive as long as dialysis cannulas can be introduced into his blood circulation. The cannulas must therefore be designed such that the blood vessels which are to be punctured are continuously treated with care. This relates both to the moment of puncture and to the entire period of time during which the cannula remains in the very sensitive blood vessel.
For reasons of economy, during largely automated production the cannulas are usually produced such that thin-walled, cylindrical high-quality tubules are initially sharpened or ground more or less obliquely to the longitudinal axis. In a subsequent working step two so-called bevels are made by sharpening the front part of the inclination to form a point of the oblique ground surface which facilitates puncturing.
The cannula is inserted such that the oval opening provided by the oblique ground surface is turned away from the surface of the skin. The following problems arise in the case of this so-called "normal cannula position" when puncturing the blood vessel and during the considerably long period in which the cannula remains in the blood vessel.
When viewed from the side, the cannula has an unequal-sided wedge shape as a result of its oblique ground surface. During puncture, or when tissue and the vessel wall are separated, force components thus act on the cannula point and tend to make the puncture path of the cannula relatively steep with respect to the longitudinal axis of the blood vessel. Whereas the resultant quick penetration into the blood vessel is desirable, further advance entails the danger of penetrating or cutting into the opposite vessel wall. The person performing the puncture must therefore exercise caution and carefully manipulate the cannula to reduce the pressure exerted on it after it has penetrated the wall of the blood vessel to prevent penetration of the opposite vessel wall. Proper performance of this technique depends considerably on the experience of the person performing the insertion.
In the "normal cannula position" the oval opening of the cannula is turned away from the skin during puncturing and therefore perceptible by the patient when the puncture is performed. When the cannula is in this position, the puncture leaves a semi-circular incision in the skin, tissue and vessel wall whose diameter corresponds approximately to the outer diameter of the cannula. In the case of thick-walled cannulas (wall thickness approximately 0.15 to 0.2 mm), the resulting tissue flap slides on the cannula wall, over the end of the ground surface and onto the cannula body. However in the case of modern cannulas with a large inner diameter and a relatively thin wall (0.05 to 0.1 mm), the tissue flap falls into the lumen of the cannula and is either cut off by the rear portion of the oval of the ground surface of the cannula (so-called "coring"), whereby the piece of tissue is punched out and possibly conveyed into the bloodstream, or the tissue flap is not cut off in the case of cannulas which are pretreated by so-called "anti-coring" at the rear of the oval, but is folded back, which is experienced as discomfort by the patient during puncturing.
A further disadvantage lies in the following. Depending upon the varying spacing between the skin surface and the blood vessel, the longitudinal axes of the cannula and of the blood vessel are always disposed, with respect to each other, at an angle which is acute to a greater or lesser degree. As a result of the eccentricity of the ground surface of the cannula, the part which projects furthest into the blood vessel, i.e. the sharp point, is next to the opposite vessel wall, as a result of which there is a danger of injury. This circumstance frequently causes perforations of the vessel wall with the formation of a local haematoma both during puncturing and when the cannula remains in the vessel for a fairly long period.
Blood vessels frequently run relatively close to the surface of the skin. In this case, the front part of the ground surface of the cannula penetrates the blood vessel while the rear oval may still lie above the surface of the skin. There is thus a connection between the blood vessel and the exterior of the skin surface and external environment via the lumen of the cannula. As a result, an amount of blood flows out onto the skin surface, particularly when the blood vessel is artificially filled to congestion, i.e., to increase in the inner pressure. In general this is at least unpleasant, but is dangerous in treatment with an artificial kidney, as many of these patients are carriers of the so-called "serum hepatitis." This illness is essentially passed on by blood or the agent of disease which is in most cases contained therein.